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Solar energy is energy obtained from the sun. It enables plants to grow and animals (including ourselves) to live. As well as this, the solar energy 'stored' by living organisms millions of years ago is released from the fossil fuels we use today. Wind is also an indirect form of solar power and the rain used in hydro-power begins as water evaporated by the sun.
When we talk about solar energy we usually mean the light or heat energy that reaches us directly from the sun. There are many ways in which we can capture and use it. These fall into one of two categories of application - solar thermal and solar electric:
Photovoltaic or solar cells generate electricity directly from solar radiation. When light energy falls on the cell, some electrons in the cell material absorb sufficient energy to break away from their atoms and establish a current flow.
Solar panels only generate electricity during the day, when the sun is shining. This means that electricity must be stored for night use and when there is cloudy weather.
Stand alone power systems are usually used where power requirements are relatively low and a grid connection or generator is not available or required. These systems use battery banks to store excess electricity generated during the day so that it can be used when your solar system is not generating sufficient power. When choosing to install a stand alone system you must ensure that it is correctly sized to ensure that a reliable power supply can be achieved. You should also remember that batteries can be expensive and will require ongoing maintenance. These systems can also be adapted to include generators which can be used to supply additional power as required. These systems are often used to supply power to remote communities.
A grid connected solar system allows you to utilise the electricity grid when your solar system is not generating enough electricity to satisfy your requirements. These systems may also give you the opportunity to sell any excess electricity you generate back to your electricity supplier. A grid connect solar system will only operate when a grid supply is present so that your system will not charge lines that maintenance crews believe are inactive. Because the solar modules are connected to the electricity grid, system size is not crucial. The main purpose of this type of solar system is to reduce your energy bills and your greenhouse gas emissions.
A. Glass Cover Plate: The cover plate is used to protect the cells from the elements. This should be kept clean in order to achieve maximum efficiency.
B. Antireflective coating: Silicon is a highly reflective material so an antireflective coating is used to reduce the losses due to reflection.
C. Contact Grid: The electricity generated by the silicon needs to be able to flow to the terminals. To help with this a grid of wire is placed on top of the silicon so that the electricity can flow through the low resistance wire instead of the silicon. A grid must be used so that most of the silicon is left exposed to the light.
D. Silicon Layers: Silicon is a semiconductor material that is used to convert energy from the sun into electricity that can be used to power our homes. The silicon is the part of the solar cell that does the work. There are two layers of silicon, each of which have slightly different properties in order to generate electricity.
E. As for D.
F. Bottom Contact: This layer works the same as the contact grid that is placed above the silicon layers. It is used to help the electricity flow to the terminals. Since the bottom of the silicon not have to be exposed to the light, the bottom contact can cover the entire surface of the silicon.
Solar thermal systems collect and store solar energy at different temperatures. Four main methods are used to concentrate the collected solar energy: parabolic troughs, parabolic dishes, power towers and solar ponds.
A solar parabolic trough generates electricity by heating fluid by concentrating the suns rays. A system of parabolic trough shaped mirrors is used to focus sunlight onto a thermally efficient receiver tube containing a heat transfer fluid. The fluid, which can reach temperatures around 400C is then used to produce superheated steam which can then be used to drive conventional turbine generators.
In order to operate efficiently the parabolic troughs must be able to adjust to ensure that the sunlight is focused on the receiver tubes. This is done using a tracking mechanism to rotate the troughs as the sun moves across the sky. Some parabolic trough systems can also rotate around a second axis to better track the sun for further improved efficiency.
These systems use a satellite dish-shaped array of mirrors to focus solar energy onto a receiver at the focal point of the dish. The receiver contains a fluid that is heated up to 1000C and is used to directly generate electricity in a small engine attached to the receiver. They track the sun on two axes, following it across the sky during the day and adjusting for the angle above the horizon which changes with season. This makes them the most effective concentrating collector.
A solar power tower system uses a circular field array of large individually-tracking mirrors (heliostats) to focus sunlight onto a central receiver located on the top of a tower. The receiver contains a fluid that absorbs the heat energy which is then used to drive a turbine generator or to provide heat directly for industrial applications. The temperature achieved by the receiver can range from 538C to 1482C. Like most solar thermal generators the heliostats are aligned using a dual-axis, computer controlled tracking system.
One factor that focusing mirrors and solar cells are restricted by is that they can not generate electricity at night unless a thermal storage system is used. Solar ponds overcome this problem by using a salt-gradient pond for thermal storage. These ponds have a high concentration of salt near the bottom, an insulating (non-convecting) middle layer and a convecting top layer with low salt concentration. Sunlight hits the pond and its solar energy is trapped in the bottom layer due to its high salinity. This heated water can not rise due to its greater density and therefore sits on the bottom heating up until it nearly boils. The middle layer acts as an insulator and prevents convection and therefore the top layers stay relatively cool. Electricity can be generated from the heated water day or night by piping a heat transfer fluid through the bottom layer for use in power generation.
Biomass & waste